The field of the invention is cooking ovens. More particularly, the invention relates to continuous-cooking ovens of the type used to commercially bake, broil, steam, or otherwise cook meats, baked goods, and other foods.
Many patents have been granted on continuous-cooking ovens. A known type of such ovens, a brander-heated oven, includes a branding unit, an elongate cooking chamber, a conveyor belt running through the chamber, and possibly steam nozzles. Brander-heated ovens aim heat from the branding unit into the product-feed end of the chamber, and this heat is the only cooking heat source for the entire oven system. The steam nozzles, if used, introduce steam into the chamber to provide humidity to the foodstuff to help achieve a higher yield. System heat is not recirculated but is instead vented at the product-discharge end of the oven. Because such ovens rely on a front-end brander as the sole heat source, such ovens experience great difficulty regulating temperature. Also, such ovens are single-unit ovens.
Another known type of continuous-cooking oven uses one or more burners, directed into the cooking chamber from the feed end, as the sole heat source. The general configuration and operation of such ovens is similar to that of a brander-heated oven, substituting the burners for the brander. Such ovens cook by direct flame, radiant heat, and hot air from the flame. Such ovens do not propel heated air through a blower into the oven chamber via nozzles directed at the food, referred to as hot-air impingement. Also, such ovens are single-unit ovens.
Another known type of oven cooks by hot-air impingement combined with a provision to recirculate vapors. Such ovens typically collect vapors at the discharge end, direct the vapors through a blower and burner, and then reintroduce the reheated vapors to the cooking chamber at the feed end or thereabouts. (While the mass of air in the oven is largely recirculated, incidental venting of some vapors may occur at the feed and discharge ends.) Such ovens may be either single units or multiple units arranged in series. Such oven assemblies are not believed to have included a front-end brander or burner as an integral part of the oven assembly. Smoke and other flavor-enriching vapors consequently do not travel the length of the cooking chamber of any oven unit to impart flavor by prolonged contact with the food.
Another known type of oven cooks by hot-air impingement in combination with feed-end burners, branders, or both, without recirculation of vapors. This type of oven was disclosed U.S. Pat. No. 5,786,566, titled “Convection/Impingement Oven For Continuously Cooking Food,” the disclosure of which is hereby incorporated by reference in its entirety.
The general features of the type of oven disclosed in the '566 patent comprise an enclosed, elongated cooking chamber, a moving belt to support the food product and carry it through the chamber, and a series of hot-air impingement units located along the length of the chamber above and below the belt. Typically, a surface-treatment section at the feed end has upper and lower burners that apply direct flame to the food product, thereby heating it, sealing it, searing it, browning it, and generally creating a desirable flavor and appearance. A rotating brander heated by the burners may apply appealing grill marks to the upper surface of the food (and the belt may apply similar marks to its lower surface). A series of steam nozzles may be located along the length of the chamber to supply water or steam to control humidity within the chamber, reduce product shrinkage, and supply moist heat for cooking.
In contrast to prior-art recirculating ovens that reuse oven air, the oven system of the '566 patent does not reuse cooking vapors. Instead, vapors pass the entire length of the chamber before being exhausted through a vent at the discharge end. Unlike recirculating ovens, ovens of the type discussed rely on an external air source to supply the air to be heated and blown through the impingement units to cook the food.
Continuous-cooking ovens of the type disclosed in the '566 patent offer very good throughput and very good control over cooking parameters such as temperature, moisture, and belt speed. Such ovens cook food by any combination of five methods—direct flame from the burners, surface cooking from the brander and belt, hot air from the impingement units, steam from the injectors, and convection from the lengthwise airflow—making them versatile. Such ovens may offer independent control above, below, and along the belt for flame, grill marking, hot air impingement, and steam injection, allowing distinct treatments for the upper and lower food surfaces. For example, the surface-treatment section typically has distinct, separately adjustable burners above and below the belt, so that flame intensity and burner angle may be controlled separately for each food surface. For another example, the upper and lower impingement units typically have separately controllable nozzles, so that hot air blown from above the food may differ in air pressure from that blown from below.
An installed oven is often part of a larger food-processing operation in which a stream of food product flows through the oven on the way to packaging and shipping. The throughput capacity of an oven is one of its most important practical properties. Capacity governs how an oven fits in with adjacent equipment and influences the throughput of the entire food-processing operation. The economics of commercial cooking often favor large-scale operations, creating an ever-present demand for higher capacity ovens to support higher production rates. The requirements of the food product usually constrain cooking time and temperature, so increasing the capacity of an oven usually dictates increasing its length. For example, suppose a chicken requires one square foot of belt space and one hour to cook. An oven with a three-foot by ten-foot belt cooks thirty chickens per hour, while an oven with a three-foot by twenty-foot belt cooks sixty chickens per hour. For continuous-belt ovens, such as the continuous-belt oven generally disclosed in the '566 patent, a higher-capacity oven is generally a longer oven.
As oven systems such as that generally disclosed in the '566 patent get longer, however, airflow within the oven can become problematic. The surface-treatment burners, impingement units, and steam nozzles constantly add hot air and steam to the cooking chamber. To maintain temperature through the length of the chamber, a longer chamber requires a larger volume of hot air, steam, or both. A longer chamber therefore has a larger volume of vapor, smoke, steam, and hot air to be exhausted at the discharge end. As the oven becomes longer, it becomes more difficult to pull air from the feed end to the discharge end. This difficulty becomes more pronounced in ovens longer than about 45 feet. A test of a 70-foot oven, for example, developing an exhaust of about 7,000 cubic feet per minute (CFM), actually lifted meat patties off the belt, through the exhaust ducting, and onto the factory roof. As this extreme example makes clear, airflow sometimes imposes a limitation on oven length and capacity.
A second issue, affecting ovens of any length, is the control of cooking parameters such as temperature along the length of the oven. Ovens of the type disclosed in the '566 patent generally have a temperature gradient running the length of the oven rather than a single fixed temperature over the entire length. At the feed end, near the surface-treatment burners, the oven may attain more than 1,500° Fahrenheit. At 45 feet, despite the ongoing addition of hot air and steam, the temperature may have fallen to about 400° F.; and at 70 feet, to about 200° F. This temperature gradient is partly the result of the airflow and exhaust issue just mentioned, since increasing temperature toward the discharge end requires increasing the amount of hot air and steam added along the chamber and exhausted at its end.
One result of the temperature gradient is restricted throughput capacity, since low temperatures toward the discharge end may force a slower conveyor rate (belt speed). Another result is reduced product yield, since a longer cooking time within the oven may increase the amount of fat, water, and other juices rendered from the food and therefore may reduce its cooked weight.
Yet another result is reduced control and versatility. High-heat transitioning to low-heat is an excellent cooking profile for many food products. That said, ovens of the type discussed have not permitted independent zonal control over temperature and other cooking parameters. Because the oven has a single, continuous cooking chamber, conditions in one region affect those of adjacent regions. Increasing the output of the surface-treatment burners to increase browning, for example, increases temperature at the feed end—but also increases subsequent temperatures, since heat from the burners travels the length of the oven. The continuous chamber limits the ability to control regions of the oven independently.
What is needed is a continuous-cooking convection/impingement oven that is scalable to higher-capacity and higher yield configurations and that offers zonal control over temperature and other cooking parameters. Ideally, the oven would be extensible to almost any length and capacity. Ideally, it would allow true, independent control over cooking conditions in a series of distinct regions arranged along the length of the oven, allowing exact tuning of processing conditions down the length of the belt.